Astron. Astrophys. 324, 988-990 (1997)

3. Discussion

In the case of V 505 Sgr the masses of all three components are known: Tomkin (1991) gives: and , Chamblis et al. (1993) estimates . Therefore - if the orbital period P is known - the semimajor axis of the orbit follows from the Kepler's law. The period is constrained by the speckle data; having in mind the constraints on the other orbital elements due to both sets of data, it proved to be impossible to represent the speckle data by a period differing from that given in Table 2 more than about 1.5 years. The inclination follows from the mass function and can be written e.g. as

The result is that the speckle measurements can be fitted only when the distance of the system from the Sun is around 100 ps (102 ps is the value for which the curve in Fig. 2 is drawn). However, the distance of the system can be reliably estimated as parsec (Tomkin 1991) using spectral classification and photometry.

Some possibilities to explain this disagreement can be suggested, it however seems to be premature to discuss them in detail until the elements are confirmed or revised be newer data.

Another discrepancy presents the difference in systemic velocity of the eclipsing pair and velocity of the third body. According to elements given in Table 2, this difference should not be larger than about 2.6 km/s in the time interval covered by observations (and the velocity of the third body should not change more than several tenths of km/s). It seems that it might be accuracy of radial velocity data which can be blamed for this disagreement (since there is no realistic third body orbit which would yield the large velocity differences published). Only during the periastron passage the difference of velocities might reach more than 10 km/s.

The orbital parameters found here must be considered as preliminary until new data are obtained. It is also necessary to point out that in Algol system, the times of minima need not to be effected by the light-time only; changes of period due to other reasons are common. To check it in the case of V 505  Sgr, new

times of minima are badly needed. Since the next periastron passage is predicted for 1998.9, they can - provided speckle interferometry and spectroscopy continue - help to find reliable values of component masses and distance of this system soon.

© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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